A typical ISDN system is illustrated in FIG. 1. The system includes external network 1 and a local network 2. The external network comprises central office 3 and a two wire "U" interface bus or line 4. The local network 2 comprises a network terminating unit (NTU) 5, a plurality of terminal equipment (TE) 6, and a four wire "S" or "T" interface bus or line. The "S" interface allows for multiple equipment (TE) to be attached in a party line configuration. Two of the four wires are used for transmission, and the remaining two are used for reception. The TEs 6 have the ability to transmit and receive data in accordance with the synchronization with network signals. In order for multiple TEs 6 to work, it is desirable that each TE 6 send the same polarity of signal to the NTU 5. When the four wire "S" interface wiring is being installed, it would be useful for the installer to know that all four wires are properly connected to each modular outlet corresponding to a potential TE 6 location without any short circuits, open circuits, or cross-connections.
A prior art system which is directed to detecting such faults in cables has been disclosed in U.S. Pat. No. 3,986,106 issued to Shuck et al. The Shuck et al. system is directed to a portable cable test set which includes a master unit connected to one end of a multiple wire cable and a remote unit connected to the other end of the cable. The pulse generator within the master unit generates a sequence of pulses of equal magnitude. The master unit gates a pulse to a preselected tip or ring wire of a predetermined cable pair. The pulse passes through the wire to the remote unit at the end of the cable where it then passes through a resistor of preselected value which is connected to the other tip or ring wire of the wire pair. A pulse comparator in the master unit compares the magnitude of the pulse return from the remote unit with the pulse passing through the reference resistance in the master unit. If the comparator determines that the pulse magnitudes are equal, the master unit then automatically sequences to the next cable pair and repeats the foregoing test. In the event of detection of unequal pulse magnitudes, a sequence interrupter in the master unit stops the test sequence, and a visual indicator identifies the wire pair having conditions activating the sequence interrupter.
Other prior art systems include U.S. Pat. No. 4,536,703 issued to Jablway et al. which discloses a complicated device for testing telephone wiring designed for use in a central office to check C.O. battery and ground on the wires. The U.S. Pat. No. 3,492,571 issued to Desler discloses a system for testing an electrical circuit for continuity, shorts and cross connections, terminals of the circuit are tested in groups, with the terminal groups being sequentially connected to the apparatus in pairs. In testing a pair of the terminal groups, a separate signal is applied to each of the terminals thereof in succession and the other terminals are monitored to detect an error in the electrical circuit. Detection of an error interrupts the testing operation and energizes circuitry for indicating the terminals involved and the nature of the defect.
However, these prior art systems fail to provide a line testing apparatus which is sufficiently portable, inexpensive, and relatively non-complicated in operation. The prior art systems for the most part do not provide line testing apparatus which is easily adaptable to home or office communication lines and which supply an easily comprehensible display of any detected faults in a particular line.
It is therefore an object of the present invention to provide a system and method for satisfying the above-mentioned needs. An installer would remove the NTU 5 and any TE 6 from the line that is desired to be tested. A signal generator would be connected to the "S" interface wiring 7, creating a distinctive signal on each line. The installer would take a receiving device, plug it successively into each modular connector, and verify that all four signals were received on the proper lines. Among the desirable criteria for the system and method were that since the "S" interface wiring could be up to a kilometer long, there should be no interconnecting wires between the generator and the receiver, other than the "S" interface bus 7 being tested. The signal generator and receiver should not be limited to require only a main source of power (120V AC), but to be open to the use of battery power. Since a NTU 5 could put 50 or more volts between the transmit and receive pairs to power a TE 6, the signal generator should be able to withstand being plugged into a live ISDN outlet. The receiver should also be able to withstand 50 volts between any lines, and ideally indicate the presence of voltage on a theoretically disconnected "S" interface bus. The unit would preferable be hand-held, portable, and manufactured and sold at a low cost for mass production. Furthermore, the information displayed to a user of the system should be relatively simple to understand, yet as informative as possible with relation to the open circuits, short circuits, and cross-connections that may have occurred in the wiring.